Hey,
Thanks for posting in the iMore Forums ell that depends the phone will lose some more value as its approaching 3 generations old now if its in good condition you can probably make some decent money off the device but I probably wouldn't expect more than a few hundred

There are people who are still using an iPhone 3G/3GS, and it's suits their needs quite well. With that in mind, the iPhone 6 Plus will still be "worth using" if it continues to suit your needs. None of us can tell you at this time how the next iPhone will be. Some people will rush to be among the first to have it. Others, like me, will probably get it, but won't rush to do so. The iPhone 6 will probably be among the devices that'll be able to run the next iOS version and so, again, if it continues to suit your needs then it will still be worth using, in my opinion.